Reading glasses 1.25 diopter strengths are bare essential wardrobe accessories once you reach the age of 40.

Then our choices expand to include bifocal glasses. If we are sun worshippers we may need to invest in a pair of bifocal sunglasses. And for convenience, folding glasses that are small and easily carried creep into our wardrobe. The choices of colors, styles and availability where we need them become important throughout the day.

What I mean by that is that the eyes begin to deteriorate for most people around this age. This condition is known as diopters, a well known term in the optical world. For example, 1 diopter is defined as the optical power of a lens with a focal length of 1 meter (about 39 inches). Since the formula is founded on the reciprocal of the focal length, this means that a 2 diopter lens equals 1/2 meter, a 3 diopter lens equals 1/3 meter and so on. This is significant because magnification increases as the focal length gets shorter.  When you need more magnification, your optometrist recommends a higher diopter correction lens or you’ll need to increase the diopters of your bifocal glasses.

40 diopters is the standard for the human eye. Over time, once we reach the age of about 25, the normal human eye is reduced to about 10 diopters.  At 50 years of age it’s a meager 1 diopter. The capacity for adjustment once the eye’s diopter gets smaller is called presbyopia.  And that is why we need reading glasses as we get older.

Reading glasses make up for the loss caused by this natural adjustment. Optical power is additive so the correction is straightforward. If you were to visit your optometrist they’d diagnose this loss and prescribes lenses to amplify the eyes power.  They usually start in quarter-diopter steps in the range of 1 to 3 diopters.

Usually a quarter-diopter is a big enough step for most people.  That’s why most people can figure out the strength of their glasses readers, folding glasses that are easy to carry, or bifocal sunglasses simply by trying on various reading glasses at the local drug store. The eye care industry categorizes reading glasses lenses in diopter strengths that increase by a factor of .25 (e.g., +.75; +1.00: +1.25; +1.50; +1.75; +2.00; +2.25; +2.50; +2.75; +3.00 etc.). So trial and error works well when you need to choose a set of reading glasses.

However, presbyopia sometimes makes it difficult to select the correct bifocal glasses. If each eye is different, for example, you may need a +1.75 for the right eye and a+2.00 for the left eye; you can balance this out by selecting the higher strength.   This is your only option if you choose to buy one-size fits all, which are most common at the drug store.

Presbyopia is the medical term for a condition that mimics farsightedness, so you may need to increase the strength of magnification in your reading glasses every year or two. It's a universal condition, and no refractive surgical procedures are available to correct it.

This condition also fluctuates based on your lifestyle and else is going on in the body.  That’s why it’s not uncommon for people to have multiple sets of reading glasses, bifocal sunglasses and folding glasses with different diopter strengths.

You don’t need a prescription to purchase reading glasses.  They are intended as an aid to help you see better at close distances.  They are not intended as a replacement for prescribed corrective lenses.  Aside from the trial and error method, at some point you should consider getting a professional eye exam from your optometrist.